The invention relates to a superconducting magnet assembly and a method for operating the assembly.
There are many applications in which superconducting magnets are used to create a stable magnetic field in a working volume. Examples include MRI, NMR, ICR and cyclotrons, in which the magnet is operated in the so-called xe2x80x9cpersistent modexe2x80x9d. This involves connecting a near zero ohm connection between the start and end of a magnet once it has been energised. The techniques for achieving this are well known. The resulting field stability is then determined by the time constant of the magnet inductance and the total circuit resistance.
The time constant is defined as L/R where L is the magnet inductance in Henries, R is the total circuit resistance in Ohms and the time constant is measured in Seconds.
So unless L=infinity or R=zero then the resulting time constant will be finite, resulting in an exponential decay of both magnet current and field with time.
Depending upon the application, it is desirable to have the decay rate as close to zero as possible, typically the NMR application would like the decay rate to be less than 0.01 ppm/hour.
For most systems the magnet inductance is fixed by the geometry required to produce the very high homogeneous field and operating current required. So, in practice, the circuit resistance of the magnet will determine the field decay rate.
Until now, this field drift has been an accepted problem and the only solution has been to reenergise the magnet.
In accordance with a first aspect of the present invention, a superconducting magnet assembly comprises a superconducting magnet which, under working conditions, generates a magnetic field in a working volume, the superconducting magnet being connected in parallel with a superconducting switch, the switch and magnet being adapted to be connected in parallel to a power source whereby under working conditions with the switch open, the magnet can be energised by the power source to generate a desired magnetic field in the working volume following which the switch is closed, and is characterised in that the assembly further comprises a resistor connected in series with the switch, the resistor and switch being connected in parallel to each of the magnet and the power source.
In accordance with a second aspect of the present invention, a method of energising a superconducting magnet assembly according to the first aspect of the present invention comprises
i) energising the magnet from the power source with the switch open;
ii) closing the switch; and
iii) changing the current supply from the power source so as to reduce drift in the magnetic field generated in the working volume.
The problems outlined above in connection with magnetic field drift are overcome with this invention by adding a resistor in series with the switch. This enables the algebraic sum of the voltages in the circuit defined by the magnet, switch and resistor to be adjusted to, or close to, zero which is the condition required for zero magnetic field drift.
In contrast to conventional systems in which the power supplied to the magnet circuit is reduced to zero once the switch has been closed, the power supply must remain connected but it is believed that the benefit of achieving substantially longer periods of stable magnetic field outweigh the cost of maintaining the power supply.
Typically, the resistor has a resistance which is at least 10-100 times larger than the resistance of the magnet although a resistance in the range 1-1000 of the magnet resistance is possible. In addition, the resistor should have substantially no inductance.
There are various methods by which the correct current to achieve zero magnetic field drift can be determined.
In the first method, the resistance of the magnet can be determined. This can conveniently be achieved by providing a second superconducting switch in parallel with the magnet and power supply, the second switch being closed once the magnet has been powered up to a required field strength; and then monitoring the magnetic field decay so as to obtain a value for the magnet resistance. The decay rate=1/time constant and the time constant also is L/R (where L is the magnet inductance and R the magnet resistance). So the magnet resistance R=decay rate (in ppm/second) multiplied by the magnet inductance L. For example, if L=100 Henries and the decay rate=3.6 ppm/hour then 3.6 E-6/3600=1E-9 seconds the inductance L=100 gives R=1E-7 Ohms.
In a second approach, a voltmeter could be mounted across the magnet and the resistance determined directly in response to the passage of a known current.
In a third approach, the method further comprises:
iv) monitoring the magnetic field decay; and, repeating steps iii-iv with a different change in current in step iii to reduce the magnetic field decay. This iterative technique avoids the need for additional components.
The magnet may have any conventional construction utilizing either or both of low temperature and high temperature superconducting materials or other materials with low bulk resistivity. Since the power supply remains connected to the magnet, high temperature superconducting current leads are preferred to reduce heat conduction and minimise heat losses in the environment.